Hillary Clinton on San Bernardino mass shooting: 'I refuse to accept this as normal'

"I refuse to accept this as normal," she wrote in a tweet after a shooting at the Inland Regional Center produced as many as 20 victims, according to the San Bernardino Fire Department. "We must take action to stop gun violence now."

Almost immediately after the shooting, which reportedly occurred at the Inland Regional Center, Maryland Gov. Martin O'Malley (D) also called the shooting "horrifying" and slammed the National Rifle Association for lobbying against gun-control legislation.

For their part, Republican presidential candidates offered thoughts and prayers for shooting victims and their families.

While all of the Republican presidential candidates have opposed proposed gun control measures, both Clinton and O'Malley have repeatedly called for gun control measures following previous mass shootings that have occurred during the 2016 primary.

In a statement to CNN in October after a shooting at a community college in Oregon, Clinton expressed exasperation with the frequency of mass shootings across the US.

"It is just beyond my comprehension that we are seeing these mass murders happen again and again and again. And as I have said, we have got to get the political will to do everything we can to keep people safe," Clinton told CNN reporter Dan Merica.

"You know I know there is a way to have sensible gun control measures that help prevent violence, prevent guns from getting into the wrong hands and save lives."